A man who "befriended" a sobbing 11-year-old girl on a train has been sentenced to jail.

A man who befriended a sobbing 11-year-old girl on a Perth train then sexually abused her has been jailed for three-and-a-half years.

Liam Jon Dreja was 32 in February last year when he struck up a conversation with the child, who was crying because she had nowhere to sleep that night, and offered to help her with accommodation.

He then took her to a primary school playground where he abused her.

District Court of WA Judge Andrew Stavrianou said the victim was clearly vulnerable and Dreja had taken advantage of her.

A psychologist told the court Dreja’s thinking and judgment had been impaired by drugs and alcohol.

“Clearly it was a very bad decision that you made on that particular evening,” Judge Stavrianou said.

“The child in this case was of tender years.”

The psychologist said Dreja’s drug dependency spanned more than 20 years and he had extensive substance abuse treatment needs.

He will be eligible for parole after serving half of his term.

If this post brings up any issues for you, or if you just feel like you need to speak to someone, please call 1800 RESPECT (1800 737 732). Those aged up to 25 years old are urged to call Kids Helpline on 1800 551 800.